This weapon is part of the set I use for my Nekros Prime Build

INTRODUCTION

Just attack every enemy and watch their health melt away.
Why? Because slash procs completely ignore armour, so you just deal true damage based on your modded base damage + multipliers. A heavy attack on a scythe will always proc slash.

BUILD SPECIFICS

INCARNON CHOICES

1. Incarnon Form: This gives us the incarnon mode and is our only option at this tier. Besides the changes the incarnon form gives we also get +100% Melee Damage, +20% Sprint Speed, and +20% Bullet Jump.
2. Swordsman's Flourish: This increase Damage by +30, which is a nice little extra. It also doubles combo chance if we want to use incarnon mode.
3. Resolute Force: This increases our combo duration to 15 seconds, even more if we decide to use arcanes like Secondary Dexterity to further boost it. Thanks to this we won't be forced to slot in Naramon as our focus school.
4. Absolute Valor: +10% flat critical chance. This heavily depends on if it'll help us break through a next crit tier or not, otherwise we'd go for status.

SLASH PROCS

To understand slash procs we have to understand all the variables. We have base damage, damage multiplier, critical multiplier, combo multiplier, and faction mods.

• BASE DAMAGE: The [Hate] has a base 230 damage on normal attacks and 1380 damage on heavy attack. On top of that we get +30 damage because of our incarnon perk so that changes the values to 260 and 1410 respectively.

• DAMAGE MULTIPLIER: For our damage multiplier we'd have +265% from [Primed Pressure Point] and our incarnon mode, so we add on the multiplier to the base damage. You should always remember that these percentages are always added on top of the base, so 100% + 265% or (1 + 265%). Only mods that increase melee damage count for this calculation, so mods like Buzz Kill would do nothing.

• CRITICAL MULTIPLIER: The calculation here is a bit complex. by default we have 7.9x from [Organ Shatter] + the riven, but I have two Tauforged Violet Archon Shards for +150% critical damage, making it a 11.6x multiplier, and if we add the flat +1.2x of our companion's [Tenacious Bond] we'll get 12.8x, but we have to keep crit chance in mind. In warframe, when a weapon has over 100% crit chance it'll always crit, but it'll have a chance to do a higher tier crit, and that keeps going on every 100% we add up.
  Crit chance is multiplied by the base and added onto it. In this case the base is 30%, and we have +220% from [Sacrificial Steel], +241.6% from the riven, and 40% per combo multiplier starting from 2x from [Blood Rush]. We also add the 10% critical chance from our perks after calculations. This makes our calculations:
  (30 x (1 + 220% + 241.6% + (40% x 11))) + 10 = 310.48% at 12x combo. This means all hits will red crit (tier 3) with a 10% chance to do tier 4 crits.
  Now, 2 of those mods double their value when doing heavy attacks, and Melee Animosity gives +42% critical chance per hit on your next heavy attack up to 420% on 10 hits [Discipline's Merit] gives Tennokai every 4 hits, however, so it's better to count that instead of the 10. That makes the calculations for heavy attacks:
  (30 x (1 + ((220% + 241.6%) x 2) + (40% x 11) + (42% x 4))) + 10 = 499.36% at 12x combo and after 4 hits. This means basically all hits will do tier 5 crits. If you stack Melee Animosity up to 10 hits the critical chance goes up to 574.96%.
  The calculation for crit multiplier is 1 + Tier x (Crit Multiplier - 1).
  So on light attacks we do 1 + (3 x (12.8 - 1)) = 36.4x damage multiplier.
  And on heavy attacks we do 1 + (5 x (12.8 - 1) = 60x damage multiplier, with a 75% chance of doing 1 + (6 x (12.8 - 1) = 71.8x if you fully stack.

• COMBO MULTIPLIER: For the combo multiplier we go up to 12x, which is easy enough considering we get double combo chance.

• FACTION MODS: Faction mods work on the same multiplier logic, so a 55% multiplier would be (1 + 55%).

DAMAGE CALCULATION

Now that we explained how to calculate every variable for the equation let's move on to the damage calculation itself.
Slash procs work on 2 equations. One for the base modded damage when doing the attack, and the other for the 6 ticks of slash you'll get after.

BASE MODDED DAMAGE:

The base modded damage equation is: Base damage x Damage multiplier x Faction multiplier.
That'd our base modded damage for light attacks would be: 260 × (1 + 265%) × (1 + 55%) = 1,470.95 damage.
And our base modded damage for heavy attacks would be: 1410 × (1 + 265%) × (1 + 55%) = 7,977.075 damage.

SLASH TICK DAMAGE:

Our slash proc tick damage is: Base modded damage x 35% x multiplied by Faction multiplier x other multipliers
That'd make our slash tick damage be:
On light attacks: 1470.95 x 35% x (1 + 55%) x 36.4 x 12 = 348,562.1958
On heavy attacks:
At tier 5 crits: 7977.075 x 35% x (1 + 55%) x 60 x 12 = 3,115,845.495
At tier 6 crits: 7977.075 x 35% x (1 + 55%) x 71.8 x 12 = 3,728,628.44235

So we're easily dealing 348k true damage per tick 70% of the time when we do light attacks (since we end up with a 100% status chance and our weapon is 70% slash damage). It'll easily 1-shot most enemies in steel path. Even if something is too stubborn to die quickly you can heavy attack for free every 4 hits with tennokai, boosting your damage up to 3.1 million true damage per tick, or a 75% chance at 3.7 million if you do 12 hits instead of before heavy attacking.

FACTION MOD REASONING:

If you didn't want to use a faction mod and replaced it with another mod that helped our variables your only other options would be [Gladiator Might] for +60% critical damage and +10% critical chance per combo multiplier. This brings our light attack crit chance to 343% while our heavy attack crit chance goes to 532% on 4 hits and 607% on 10 hits. As for the rest of our calculations:

Base modded damage:
260 × (1 + 265%) = 949
1410 × (1 + 265%) = 5,146.5
Slash tick damage:
On light Attack: 949 x 35% x 36.4 x 12 = 145,083.12
On heavy attack:
At tier 5 crits: 5146.5 x 35% x 60 x 12 = 1,296,918
At tier 6 crits: 5146.5 x 35% x 71.8 x 12 = 1,551,978.54

As you can see we lose over half our damage removing a faction mod, even if we try making up for it. That's how important faction mods are when considering this.

BUFF ABILITIES AND FACTION MOD ALTERNATIVE:

Regarding damage buffs your main options are Eclipse, Xata's Whisper, and Roar.

• Eclipse does the same damage as Roar, but it doesn't have the unique interaction the other ability has, so because of that it's out.
• Xata's Whisper doesn't affect our bleed procs at all, since it just creates a separate instance of void damage duplicated from the initial hit and double dipping on faction mods again.
• Roar works in the same way as a faction mod would, so it's best used by replacing the faction mod with it.
  At 100% ability strength Roar gives a 30% multiplier (equivalent to non-primed faction mods).
  At 184% ability strength Roar gives a 55% multiplier (Equivalent to primed faction mods).
  Any strength after that just increases the overall calculations. For example; at 270% ability strength Roar gives a 81% multiplier, which would make our calculations before give us:
  Base modded damage:
  260 × (1 + 265%) × (1 + 81%) = 1,717.69
  1410 × (1 + 265%) × (1 + 81%) = 9,315.165
  Slash tick damage:
  On light attacks: 1717.69 x 35% x (1 + 81%) x 36.4 x 12 = 475,306.809432
  On heavy attacks:
  At tier 5 crits: 9315.165 x 35% x (1 + 81%) x 60 x 12 = 4,248,833.0598
  At tier 6 crits: 9315.165 x 35% x (1 + 81%) x 71.8 x 12 = 5,084,436.89489
  Which on top of being an overall buff, it also opens up one of our mod slots, letting us put things like attack speed on it.

OTHER ADDITIVE DPS BOOSTING SOURCES:

ATTACK SPEED:

• [Primed Fury] or [Berserker Fury]: The first of the two works at all times, while the second takes a bit longer to trigger, but as soon as it does it ends up outdamaging the first. The main issue with these is that we're rather tight in mod slots. If you don't have a riven or don't like switching around faction mods then you could replace either of those with either of these.
• Arcane Strike: If you happen to have a free arcane slot this is a great alternative, with a +60% attack speed buff at rank 5, it seems to trigger almost all the time in an actual mission.
• Boosting abilities: Warframes like [Gauss Prime] or Wisp Prime can boost their attack speed substantially, and thus don't require any other methods to do so.

DAMAGE:

• Void Strike: The Madurai focus school gives us this ability. When cast we get up to +1000% damage at full energy. On top of that Phoenix Talons gives us +30% damage passively. That'd make our original calculations be:
  Base modded damage:
  260 × (1 + 265% + 1030%) × (1 + 55%) = 5,621.85
  1410 × (1 + 265% + 1030%) × (1 + 55%) = 30,487.725.
  Slash tick damage:
  On light attacks: 5621.85 x 35% x (1 + 55%) x 36.4 x 12 = 1,332,176.0634
  On heavy attacks:
  At tier 5 crits: 30487.725 x 35% x (1 + 55%) x 60 x 12 = 11,908,505.385
  At tier 6 crits: 30487.725 x 35% x (1 + 55%) x 71.8 x 12 = 14,250,511.444
  As you can see the damage almost quadruples, but sadly this ability lasts only for a few seconds with a really long cooldown. You'd only able to do 2 tennokai heavy attacks or so at most before it runs out.

CRITICAL CHANCE:

• Arcane Avenger: This warframe arcane gives you a flat 45% crit chance, and its effect triggers often enough while being hit that you'd have it up most of the time under enemy fire. Flat bonuses are added after calculations, so if your weapon had a base 20% crit chance and a total 155% crit chance after mods, this would bring your weapon to 200% crit chance, instead of 164%. It's a great addition to bring your crit chance to the next tier if your warframe of choice can take the hits. In our case it'd bring our chance of tier 4 crits from 7% to 52%, giving us tier 4 crits half the time.
• [Cat's Eye]: This [Adarza Kavat] mod gives you a flat 60% crit chance for 10 seconds (13 if using [Tek Enhance]) with a 20 second cooldown after the buff runs out. We already discussed why flat crit is great, so I won't repeat that. While the crit chance is even better than Arcane Avenger it suffers from 2 major drawbacks. The first is the 20 second cooldown, effectively making it so that the buff is only active around 33% of the time. The second is that it's an [Adarza Kavat] mod, and at higher levels your companion falling down way faster, so it'd give you one more thing to worry about.

RIVEN:

The riven is quite important to this build, sadly. The only replacement is [Gladiator Might], and even then it's a substantial loss. All in all it'll still deal disgusting amounts of damage, but not AS disgusting.